Diffusive processes in aqueous glass dissolution

Thomas L Goût,Rui Guo,Sambuddha Misra,Ian Farnan,Madeleine S Bohlin,Edward T Tipper

doi:10.1038/s41529-019-0102-5

Thomas L Goût, Rui Guo + Show 4 more

Open Access

https://doi.org/10.1038/s41529-019-0102-5

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Abstract

High level nuclear waste is often immobilised in a borosilicate glass for disposal. However, this glass corrodes in contact with aqueous solutions. To predict radionuclide releases from wasteforms, their dissolution mechanisms must be understood. Understanding glass dissolution mechanisms presents a challenge across numerous other disciplines and many glass dissolution models still remain conflicted. Here we show that diffusion was a significant process during the later stages of dissolution of a simplified waste glass but was not evidenced during the initial stages of dissolution. The absence of measurable isotopic fractionation in solution initially supports models of congruent dissolution. However, the solution becoming isotopically lighter at later times evidences diffusive isotopic fractionation and opposes models that exclude diffusive transport as a significant mechanism. The periodically sampled isotopic methodologies outlined here provide an additional dimension with which to understand glass dissolution mechanisms beyond the usual measurement of solution concentrations and, post-process, nano-scale analysis of the altered glass.

Highlights

The safe disposal of vitrified nuclear waste arising from the reprocessing of spent nuclear fuel requires that dissolution rates within geological disposal facilities are accurately modelled and aqueous dissolution mechanisms are well understood.[1]
Considering a timespan of at least one million years,[8] the rates of residual glass alteration would be different if dissolution was congruent through network hydrolysis from a supersaturated interfacial film of water, hypothesised by the interfacial dissolution-precipitation model,[3,4] or if dissolution was incongruent as predicted by diffusion-based models; some of which hypothesise interdiffusion reactions and the precipitation of secondary phases control the residual rate of glass dissolution.[1,7,9]
B is known to adsorb onto the surface of clays with a strong pH dependency,[27] significant masses of B are not expected to co-precipitate from solution during glass dissolution.[28]

Summary

Introduction

The safe disposal of vitrified nuclear waste arising from the reprocessing of spent nuclear fuel requires that dissolution rates within geological disposal facilities are accurately modelled and aqueous dissolution mechanisms are well understood.[1]. In the weathering of natural systems, Li potentially fractionates through two isotope effects: kinetic isotope effects during dissolution of the primary phase or equilibrium isotope effects where Li is incorporated into or adsorbed onto secondary phases.[10,11,12] Kinetic isotope effects are associated with rapid, unidirectional processes wherein the lighter isotope is preferentially transported across a phase boundary (such as in diffusion) or through a phase (such as in evaporation) due to its higher velocity.[13] While the large mass difference between 6Li and 7Li (~15%) should be very sensitive to kinetic isotope effects, no such effects have been observed during the dissolution of the primary phase during basaltic glass dissolution.[11,14,15,16] Equilibrium isotope effects occur due to differences in isotopic masses effecting differences in bond energy preferences. For Li, longer bond lengths, less stiff bonds and lower bond energies are associated with higher coordination number sites.[10,13] As such, when there is precipitation, 6Li is preferentially partitioned into the higher CN octahedral and pseudo-hexagonal sites (CN of six) of clay secondary phases, 7Li preferentially remains in solution (CN of four)[10,18,19] and the solution becomes isotopically heavier with time.[20,21,22,23,24] Further, Li can be adsorbed into the interlayer sites of smectites or onto the adsorption sites of gibbsite and ferrihydrite, the former is not expected to be associated with any isotopic fractionation.[10,14,18]

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